The subject invention relates generally to a method and a device for recovery of energy from media containing combustible substances even at low-concentrations, and more particularly to energy recovery in connection with the purification of said media by combustion of the combustible substances.
It is already known to purify exhaust fumes emanating for instance from painting and printing works and containing combustible substances, such as solvents, by heating the fumes up to the temperature of combustion of the combustible substances. However, this is an expensive process on account of the need to heat the entire flow of the exhaust medium. Owing to a "combustion exchanger", one embodiment of which is described for instance in connection with an absorption technique in the Swedish Patent Specification 8903556-2, it is possible to reduce the costs associated with heating the exhaust fumes. In said Patent Specification the combustion exchanger is defined as an apparatus comprising a heat-accumulating bed which may be heated up to the spontaneous combustion temperature of the combustible substances and in which bed the combustible substances, by reversing their direction of flow, attribute to sustaining the spontaneous combustion temperature inside the bed. An apparatus of this type is described also in e.g. U.S. Pat. No. 4,741,690. When certain amounts of the combustible substances are present in the medium the heating of the medium inside the combustion exchanger may take place by the energy released from the spontaneous combustion of the substances inside the bed. Supplementary heating thus becomes necessary only to heat the bed to the spontaneous combustion temperature when the plant is being started up or when the quantity of combustible substances is very small and insufficient to independently sustain the spontaneous combustion temperature inside the bed.
Owing to the high efficiency of the combustion exchanger it is, however, comparatively rare that the amount of combustible substances in the medium is insufficient, as in the latter case mentioned above. Thus, the amount of combustible substances usually is in excess of the amount required to sustain the spontaneous combustion temperature inside the bed. This leads to an increase of the heat in the bed, which in turn results in an increased demand for cooling of the bed in order to avoid damages thereto and to the surrounding area. In addition, the fact that the thermal energy developed in the combustion is not recovered is a problem as such, considering the high energy costs and environmental aspects.